1. Field of the Invention
The present invention relates to an oil pressure control apparatus of a vehicle power transmission apparatus.
2. Description of the Related Art
As a power transmission apparatus provided in a vehicle, a power transmission apparatus is known that has a belt-driven continuously variable transmission in which a belt is clamped with oil pressure to transmit motive power, and the gear ratio is changed by changing the belt contact radius, a hydraulic friction engaging element for travel (for example, a forward movement clutch) that is engaged in order to establish a motive power transmission path when the vehicle travels, and a hydraulic lockup clutch that is provided in a hydrodynamic motive power transmission apparatus provided in the motive power transmission path and directly couples a motive power source side and the belt-driven continuously variable transmission side.
In an oil pressure control apparatus of this sort of vehicle power transmission apparatus, many control valves of various types, electromagnetic valves that control those control valves, and the like are provided. For example, valves provided include a line oil pressure control valve that regulates a line oil pressure that becomes the source pressure for the oil pressure of various parts; a gearshift oil pressure control valve that regulates the line oil pressure that becomes the source pressure, and supplies a gearshift oil pressure that controls a gear ratio of the belt-driven continuously variable transmission to a driving-side pulley (primary pulley) of the belt-driven continuously variable transmission; a clamping oil pressure control valve that likewise regulates the line oil pressure that becomes the source pressure, and supplies a clamping oil pressure that controls belt clamping of the belt-driven continuously variable transmission to a driven-side pulley (secondary pulley) of the belt-driven continuously variable transmission; a garage control valve that can switch an engaging oil pressure, which is supplied when engaging the friction engaging element for travel, between an engaging transition oil pressure and an engagement holding oil pressure; and a lockup control valve that is switched when performing control to engage/release the lockup clutch. Also provided are linear electromagnetic valves, ON-OFF electromagnetic valves, or the like for controlling each of these control valves.
Conventionally, control of the line oil pressure control valve is performed with either one of two control oil pressures: the control oil pressure of a first linear electromagnetic valve that controls the clamping oil pressure control valve, and the control oil pressure of a second linear electromagnetic valve that controls engagement/release of the lockup clutch. Specifically, by switching an oil path according to engagement/release of the lockup clutch, the control oil pressure of either one of the linear electromagnetic valves is supplied to the line oil pressure control valve. In this case, the line oil pressure is controlled with the first linear electromagnetic valve when engaging the lockup clutch, and controlled with the second electromagnetic valve when releasing the lockup clutch. On the other hand, JP 2000-130574A discloses technology in which only control of the line oil pressure control valve is performed with a single linear electromagnetic valve.
Incidentally, when the control oil pressure of the linear electromagnetic valve drops due to a failure of the linear electromagnetic valve or the like, there is a possibility that a condition will occur in which the line oil pressure that becomes the source pressure is inadequate. As a result, on a low gear side where the gear ratio of the belt-driven continuously variable transmission is large, it is difficult to secure the clamping oil pressure necessary for belt clamping, so there is a possibility that belt slippage will occur.
When controlling the line oil pressure control valve by switching the oil path according to engagement/release of the lockup clutch as described above, in the case of engaging the lockup clutch and in the case of releasing the lockup clutch, the line oil pressure control valve is controlled with only respective independent linear electromagnetic valves, so there is a high possibility that the line oil pressure will be inadequate and thus belt slippage will occur.
On the other hand, when a dedicated linear electromagnetic valve that controls only the line oil pressure control valve is provided, the number of electromagnetic valves increases, resulting in increased size and cost of the apparatus.